The present invention relates to a dynamic light scattering (DLS) or photon correlation spectroscopy (PCS) method and apparatus, particularly but not exclusively for Fluorescence Correlation Spectroscopy (FCS) or lidar applications.
PCS/DLS involve the illumination of a sample with coherent light. The intensity of photons scattered from the sample fluctuates as a result of the Brownian motion of scattering particles in the sample. The number of photons scattered in a narrow range of angles, defined by specific apertures and light propagating and focusing elements, is detected as a function of time and the resultant function is autocorrelated, preferably in real time. This allows the distribution of relaxation time constants and therefore the distribution of sizes of the scattering particles to be determined.
FCS involves the use of a similar technique on fluorescent samples, except that the photons emitted by fluorescence, instead of scattered photons, are detected as a function of time. The resultant function is autocorrelated in real time, in order to measure the specific change in binding of molecules, detect molecules present in low concentrations and determine related reaction rates.
DLS-Lidar involves the active remote sensing of a portion of the atmosphere by illuminating the portion with coherent light and detecting the intensity of photons scattered from that portion as a function of time. The resultant function is autocorrelated in real time, in order to derive the particle size distribution within the sampled portion of the atmosphere.
A problem associated with the above techniques is that, as the number per unit volume of scattering particles increases, there is an increased probability of multiple scattering in which a detected photon has been scattered by more than one particle, which means that the desired properties can no longer be measured directly. Furthermore, the range in velocity of Brownian motion becomes restricted because of collisions between scattering particles. In order to compensate for these effects, the normal practice is to detect the scattered photon intensity at a range of different scattering angles, typically from 10xc2x0 to 180xc2x0 (backscattering). As there is a quadratic dependence of time constants on scattering angle for the ideal case, any deviation from this quadratic function, due to multiple scattering or restricted particle collision, is quantifiable and can be compensated for. However, such procedures are time-consuming because of the number of readings that need to be taken, and require expensive goniometers.
In lidar applications, it is not practically feasible to perform angle-dependent scans of a portion of the atmosphere, so that the particle size distribution and degree of multiple scattering cannot readily be measured.
According to one aspect of the present invention, there is provided a dynamic light scattering apparatus and method in which a sample is illuminated by laser light of different wavelengths and the fluctuations in light scattered or fluoresced by the sample at each wavelength are detected. The time constants of particles in the sample are derived from the detected light fluctuations by auto-correlation or cross-correlation of the detected light intensity with respect to time. In one aspect, measurement of light scattering or fluorescence at different wavelengths is normalised to equivalent respective scattering angles, so as to give precise information on properties such as particle cross-sections or multiple scattering aspects. In another aspect, illumination at each wavelength takes place substantially along a first axis, and scattered radiation at each wavelength takes place along a second axis.
According to another aspect of the present invention, there is provided a method and apparatus for measuring the velocity of particles in a sample by means of dynamic light scattering, in which the frequency of oscillations in correlation functions of the detected intensity fluctuations as a function of time are measured and used to determine components of velocity in the direction of detection.
According to another aspect of the present invention there is provided a method and apparatus for the measurement of properties of particles in a section of atmosphere by means of dynamic light scattering, in which the section is illuminated with laser pulses and the pulses scattered from that section are detected and their intensity fluctuation correlated to determine the properties of the particles. Preferably, the intensity fluctuations of the detected pulses are concatenated and/or high-pass filtered prior to correlation. In this way, dynamic light scattering techniques may be applied to remote sensing of the atmosphere.
In an embodiment of the invention, a dynamic light scattering apparatus comprises discrete numbers of transmitting lasers with various wavelengths ranging from the visible to the near infrared. Each discrete laser transmitter forms a transmitter channel, which operates in conjunction with one or more receiver channels. The laser light propagates to the light scattering sample via apertures, focusing, telescope and/or fibre optical elements. By these elements, beam waists are produced in the sample. Corresponding to the channels of transmitting lasers are a second set of functionally identical elements, for collecting and receiving each wavelength of scattered light separately and simultaneously. The apertures, light collecting and propagating elements for one channel are adjusted in combination to produce an intercept of the dynamic part of the autocorrelation function close to unity.
The automated functions of the apparatus from one back scattering position replace the function of a goniometer for scattering angle scans. This is achieved by (quasi-) simultaneous, n-fold channel operation. The apparatus enables classification and quantification of multiple scattering sample systems, the normalisation of particle cross-sections, sizes and shapes to the wavelengths used and the quantification of repulsive forces exerted to particles or restricted particle collision as in a gel-like system with cage functions. If multiple scattering is encountered the apparatus allows adjustments to be selected to derive the true particle size. The simultaneous multiple volume, n-fold, acquisition capability of the apparatus applies to three general fields: particle sizing in the atmosphere by active remote sensing, particle sizing and visco-elastic property determination in fluids and suspensions as for example in crystal growth of bio molecules from solutions and property determination of light emitting samples by fluorescence recording as for example in immune assay developments. The apparatus utilises cross-correlation, depolarised and polychromatic dynamic light scattering in pulsed and continuous wave operation and filtering methods to derive appropriate autocorrelation functions.